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The Lower Silurian Formations of Wisconsin and Minnesota Compared

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The Lower Silurian Formations of Wisconsin and Minnesota Compared Journal of the Minnesota Academy of Science Volume 3 Number 3 Article 4 1891 The Lower Silurian Formations of Wisconsin and Minnesota Compared F. W. Sardeson Follow this and additional works at: https://digitalcommons.morris.umn.edu/jmas Part of the Geology Commons Recommended Citation Sardeson, F. W. (1891). The Lower Silurian Formations of Wisconsin and Minnesota Compared. Journal of the Minnesota Academy of Science, Vol. 3 No.3, 319-326. Retrieved from https://digitalcommons.morris.umn.edu/jmas/vol3/iss3/4 This Article is brought to you for free and open access by the Journals at University of Minnesota Morris Digital Well. It has been accepted for inclusion in Journal of the Minnesota Academy of Science by an authorized editor of University of Minnesota Morris Digital Well. For more information, please contact [email protected]. Lo·wcr Silurian of Wisconsin and Jvlim~esota. 319 The fossils are for the most part, marked out by discolora· tion (brown or red), but a few by cleavage only. They are quite numerous and are easily found when one once knows how and where to look for them. They occur fifty or more feet below the top of th~ formation. I have assigned the specimens found, to the genera anu species to which I think they belong. They are remarkably like species found in the lower part of the Trenton shales and in the Trenton limestone which here rests conformably on the Saint Peter sandstone. And it may be, as has been suggested, that, the Saint Peter is of the Silurian rather than that of the Cambrian formation. As soon as spring opens, I shall spend some days in a more thorough search, in order to find out as far as possible, the true nature and horizon of these fossils in the Saint Peter sandstone. February 3, 1891. THE LOWER SILURIAN FORMATIONS OF WISCONSIN AND MINNESOT.\ COMPARED.-F. W. Sardeson. It is the purpose of this paper to give some observations on the Silurian of Minnesota, and the Trenton group in particular; and to compare it with the same of Wisconsin. There are some difficulties in undertaking such a comparison. For example, the Trenton group in Wisconsin is nearly all lime· stone, while in Minnesota it is largely composed of shales. This lithological difference is accompanied by some differences in the fauna and in the outward appearance of the fossils. Then, too, four beds are recognized in the Trenton of vVisconsin, the Lower Buff, Lower Blue, Upper Buff and Upper Blue beds, while in Minnesota two are usually spoken of-Trenton limestone, or shell beds, and Trenton shales, or green shales. These difficulties I shall aim to a'·oid in part and in part explain. I shall take up one by one the beds as seen in Minnesota and compare them with the same in vVisconsin, so far as I can. The lower Trenton limestone, or· Trenton limestone of :Min· nesota, consists of three beds differing somewhat in lithological character and fauna; most strongly so in the area around the "Twin Cities," i. c., Minneapolis and Saint Paul. The first of these, next to and conformable with the Saint Peter sandstone, is the same bed as the Lower Buff limestone of Wisconsin, judg- Digitized by Coogle 320 Lo<;.•a Silurian of Wisconsin a11d .lliuucsota. ing from its fauna, a variety of Orthis suboeqttata Con., 0. de­ flecta Con., Rhynchonel/a orienta/is Bill., Strophomena mimuso­ tensis Winchell (varieties), Ambonychia attmuata Han, etc. At Janesville, Wis., this bed is about eighteen feet in thick­ ness, in Jefferson county, Wis., about fourteen feet; in LaFayette Co., ten feet; at Dodgeville and Platteville, Wis., nearly twent\' feet. In Fillmore Co., Miimesota, it is about twelve feet; at Rochester, Olmsted Co., the same; at Minneapolis, fifteen feet; at Faribault, Rice Co., it is either represented by four and one­ half feet of green (apparently unfossiliferous) shale, or it is absent. The .second of the three limestone beds at :\linneapolis (seven feet of carbonaceous limestone) preserves few fossil3 well. In nearly an other respects it is like the bed above rather than the one below it. But I am not so sure that this is the case elsewhere. At Faribault the seven feet immediately on the green shale mentioned above has characteristics of this bed. In south· eastern Minnesota it is less easily distinguishable from the bed be­ low. In southwestern Wisconsin I could not find it at all, unless it is there more fossiliferous, and hence confused with the strata above. But at other points in Wisconsin, these strata seem to be distinguishable though in every case less distinctly than at Min­ neapolis. The five feet three inches at the top of the limestone at l\Iin­ neapolis is the fossiliferous bed of the three. The species that occur in it are essentially the same fonns as those common in the Lower Blue bed of \Visconsin. Orthis pervcta Con., Trochonema beloitellse \Vhitf., Cypricardites rectirostris H., are abundant fonns. At Faribault this bed is of about the usual depth for Minne­ sota, but is darker and more carbonaceous than usual and brachio­ pod shells are well preserved. In Wisconsin, at Platteville, ~Iin­ eral Point and Dodg-eville, it seemed to be from ten to fifteen feet thick and perhaps the same thickness at Janesville. The rest of the Lower Trenton in Minnesota is !'hale which is supposed to· be mainly the equivalent of the Cpper Buff an<l Upper Blue limestone of Wisconsin. But the lithological differ­ ences, together with the scarcity of fauna in Wisconsin make this more difficult to decide. All the evidence met with, however. is in favor of the supposition that the lower Trenton shales in Min- Digitized by Coogle Lo'i;.'cr Silurian of Wisconsin and .llfiunesota. 32I nesota are the equivalents of the Upper Buff and Blue beds in Wisconsin, with the exception of the first ten feet of our shales. These last are, doubtless, a part of the Lower Blue bed in Wis­ consin. I came to the conclusion, some time ago, that the first strata of the shales could be classed with the limestone below, as easily as with the shales above. They form here a transitional bed, which for convenience I wish to distinguish as the Stictoporel/a bed. The limestone strata, which constitutes part of it, though crystalline like the slabs in the true shales above, are the result of sedimentation like the limestones below. The fauna, too, is as much that of the preceding as of the succeeding strata. The Stictoporclla bed, in ascending order, is as follows: Limestone 6 in., limestone I ft. I in., limestone 2 ft., shale I ft. 6 in., limestone 7 in., shale 5 ft., lime!'tone I ft. 6 in. (measure­ ments taken at Saint Paul.) The shaly parts are not unmixed day, but have numerous thin hard calcareous laminre in them. The stone and shale vary locally in thickness and alternation, but are of about the same proportion, as seen in Goodhue, Olmsted and Fillmore counties, Minnesota. There is a bed of dark colored limestone upon the Lower Blue bed at Platteville, Wis., which appears to be the same as the Stictoporella bed in Minnesota. It consists of, first, about four feet of solid strata, with thinner clea,·ablc strata of the same col­ or; second, three to four feet of green shale such as commonly occurs in ~dinnesota; and third, four feet of dark colored stone, apparently the transitional back to the ordinary limestone. The fossils were most of them characteristic forms of the Stictoporella bed in Minnesota. At Dodge,·ille, Wis., the same strata, so far as I could judge, occur: as a light brown bed about ten feet thick, but quite unfos­ siliferous. At the time this place was examined, I was very much puzzled as to whether this bed belong-ed to the Lower Blue or l'pper Buff limestone, but upon reading over the Geology uf \Vis­ consin, Vol. I, I became quite satisfied that it would be classified as equivalent to part of the Lower Blue limestone of the Rock river valley. Along the Illinois Central railroad lll'ar Dodgeville, nearly every stratunf from the Saint Peter to the middle of the Galena is clearly exposed. There can be r{'('og-nized the Lower Buff, Digitized by Coogle 322 Lower Silurian of Wisconsin and Mim1csota. Lower Blue including the bed mentioned above, the Upper Buff and Upper Blue beds. But neither there nor in the Rock river valley do any strong evidences appear to prove that the Upper Buff and .Upper Blue beds are equivalent to the Trenton shales of Minnesota. Only their position suggests that they are very probably equivalents. The Trenton shales are about eighty feet thick at Saint Paul. In Goodhue county they are about the same. In Fillmore county they are much thinner mainly on account of the upper strata, as exposed in Saint Paul, being here represented by fifteen or twenty feet of limestone. So far as known the beds of the shales were co-extensive in deposition in Minnesota. The first ten feet of Trenton shales has been described above as the Stictoporella bed. It is very fossiliferous; Orthis sub­ aequata var., gibbosa Bill., Anoloteichia impolita Ulr., Pachydictya foliata Ulr., and Stictoporella frondifcra Ulr., occur in masses. The last named is unknow~ except in this bed and is very widely distributed; and for that reason the name Stictoporella has been proposed for this ten feet of shale. The next thirty feet is of unifom1 dark green unctuous shale with numerous fossils, but many of them poor on account of the nature of the matrix.
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